Open Access Highly Accessed Research article

Human Streptococcus agalactiae strains in aquatic mammals and fish

Christian MJ Delannoy12, Margaret Crumlish1, Michael C Fontaine2, Jolinda Pollock24, Geoff Foster3, Mark P Dagleish2, James F Turnbull1 and Ruth N Zadoks25*

Author Affiliations

1 Institute of Aquaculture, School of Natural Sciences, University of Stirling, Stirling, UK

2 Moredun Research Institute, Pentlands Science Park, Bush Loan, Penicuik, UK

3 SAC Consulting Veterinary Services, Drummondhill, Stratherrick Road, Inverness, UK

4 Current address: SRUC (formally SAC), The Roslin Institute, The University of Edinburgh, Easter Bush, Midlothian, UK

5 Current address: Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, Scotland, UK

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BMC Microbiology 2013, 13:41  doi:10.1186/1471-2180-13-41

Published: 18 February 2013



In humans, Streptococcus agalactiae or group B streptococcus (GBS) is a frequent coloniser of the rectovaginal tract, a major cause of neonatal infectious disease and an emerging cause of disease in non-pregnant adults. In addition, Streptococcus agalactiae causes invasive disease in fish, compromising food security and posing a zoonotic hazard. We studied the molecular epidemiology of S. agalactiae in fish and other aquatic species to assess potential for pathogen transmission between aquatic species and humans.


Isolates from fish (n = 26), seals (n = 6), a dolphin and a frog were characterized by pulsed-field gel electrophoresis, multilocus sequence typing and standardized 3-set genotyping, i.e. molecular serotyping and profiling of surface protein genes and mobile genetic elements.


Four subpopulations of S. agalactiae were identified among aquatic isolates. Sequence type (ST) 283 serotype III-4 and its novel single locus variant ST491 were detected in fish from Southeast Asia and shared a 3-set genotype identical to that of an emerging ST283 clone associated with invasive disease of adult humans in Asia. The human pathogenic strain ST7 serotype Ia was also detected in fish from Asia. ST23 serotype Ia, a subpopulation that is normally associated with human carriage, was found in all grey seals, suggesting that human effluent may contribute to microbial pollution of surface water and exposure of sea mammals to human pathogens. The final subpopulation consisted of non-haemolytic ST260 and ST261 serotype Ib isolates, which belong to a fish-associated clonal complex that has never been reported from humans.


The apparent association of the four subpopulations of S. agalactiae with specific groups of host species suggests that some strains of aquatic S. agalactiae may present a zoonotic or anthroponotic hazard. Furthermore, it provides a rational framework for exploration of pathogenesis and host-associated genome content of S. agalactiae strains.

Streptococcus agalactiae; Fish; Sea mammal; Aquatic epidemiology; Molecular epidemiology; MLST; PFGE